Synthesis and characterization of some derivatives of 1,3-Diisopropyl-4,5- dimethylimidazol-2-ylidene

Article history: Received February 28, 2020 Received in revised form April 9, 2020 Accepted April 9, 2020 Available online April 10, 2020 N-Heterocyclic carbenes are widely used in organic reactions and coordination chemistry. In the present study, 2,3-dihydro-1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (1) is reacted with diphenyl disulfide, methyl phenyl disulfide, and bis(methylsulfonyl)methane to yield target compounds 5, 6, and 7 respectively. Structures of these compounds are well established using nuclear magnetic resonance, mass spectrometry and elemental analysis. Possible reaction mechanisms are proposed. © 2020 Growing Science Ltd. All rights reserved.


Introduction
N-Heterocyclic carbenes (NHCs) have played an important role in various fields of chemistry, including medicinal chemistry, transition metal catalysis, and material chemistry. [1][2][3] More specifically, NHCs have recently received significant attention for the development of materials and novel drugs. 1,3 Further, NHCs are proven major ligand class. 2 On the other hand, a 2,3-dihydro-1,3-diisopropyl-4,5dimethylimidazol-2-ylidene (1) has a strong basic character and consequently can form various imidazolium salts (2) [4][5][6] , and in parallel it poses a good nucleophilic property to form new derivatives (3)(4). [7][8] There is a much interest in imidazolium salts based on their uses as ionic liquids. [9][10] Expanding our systematic study on heterocyclic carbenes and continuing our investigations on the chemistry of imidazol-2-ylidene, we report herein its reactions with diphenyl disulfide, methyl phenyl disulfide and bis(methylsulfonyl)methane. To the best of our knowledge, none of these reactions have been reported previously.
Compound 5 was characterized by NMR and IR spectroscopy, mass spectrometer and elemental analysis. NMR and mass data are in good agreement with those published in the literature. 11,12 Mechanism of synthesis is proposed in Scheme 2. It seems that C2 of compound 1 attacks a sulfur atom in methyl phenyl disulfide that is less-hindered followed by attacking the sulfur atom of thiophenolate to the methyl group under SN2 mechanism to produce the target product 5. This sulfur-sulfur bond cleavage mechanism was observed in selective desulfurization of trisulfides. 13 Scheme 2. Proposed mechanism for synthesis of 5 The structure of compound 6 was assigned obviously from data of NMR and IR spectroscopy, mass spectrometry and elemental analysis. Diphenyl disulfide shows only four signals in the 13 C NMR spectra due to the presence of symmetry between phenyl groups, while in 6 the symmetry between the two phenyl rings have been disappeared due to the cleavage of S-S bond and formation of the salt. In addition, all the imidazolium ion signals are existing in the expected range. 1 H and 13 C NMR data of 6 imply the presence of separated ions. A proposed mechanism for synthesis of compound 6 is shown in Scheme 3; carbon atom (S-C) of the phenyl group in imidazolium cation cannot be attacked by sulfur atom of thiophenolate anion due to the electronic and steric effects.

Scheme 3.
Proposed mechanism for synthesis of 6 2,3-Dihydro-1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene is considered a strong organic base, and consequently can be employed as a deprotonation reagent 14 to form imidazolium compounds which have an important role in developing ionic liquids. These liquids were applied as pharmaceutical solvents. 15 In the present study, the reaction of compound 1 with bis(methylsulfonyl)methane (Broenstedt acid) represents an acid-base reaction.
imidazoliumacetylacetonate (acac) salt showed a significant downfield chemical shift in 13 C spectrum for the CHacac (δ = 101.7 ppm). 17 Comparing with the present value (63.01 ppm), this difference might be attributed to the electronegativity difference of sulfur and oxygen atoms. All attempts to get single crystals from 7 were failed due to the very low stability of the salt and high sensitivity towards the moisture.

Conclusion
Target compounds 5, 6, and 7 were prepared successfully in a reasonable yield from the reaction of 2,3-dihydro-1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (1) with diphenyl disulfide, methyl phenyl disulfide and bis(methylsulfonyl)methane respectively. Structures of these compounds were fully characterized using various spectroscopic techniques. Compound 1 may act as good nucleophile and strong base in various organic reactions under dry conditions.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.